Router

A router is a device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers are located at gateways, the places where two or more networks connect, and are the critical device that keeps data flowing between networks and keeps the networks connected to the Internet. When data is sent between locations on one network or from one network to a second network the data is always seen and directed to the correct location by the router. They accomplish his by using headers and forwarding tables to determine the best path for forwarding the data packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts.
The Internet itself is a global network connecting millions of computers and smaller networks — so you can see how crucial the role of a router is to our way of communicating and computing.

Why Would I Need a Router?
For most home users, they may want to set-up a LAN (local Area Network) or WLAN (wireless LAN) and connect all computers to the Internet without having to pay a full broadband subscription service to their ISP for each computer on the network. In many instances, an ISP will allow you to use a router and connect multiple computers to a single Internet connection and pay a nominal fee for each additional computer sharing the connection. This is when home users will want to look at smaller routers, often called broadband routers that enable two or more computers to share an Internet connection. Within a business or organization, you may need to connect multiple computers to the Internet, but also want to connect multiple private networks — and these are the types of functions a router is designed for.

Routers for Home & Small Business
Not all routers are created equal since their job will differ slightly from network to network. Additionally, you may look at a piece of hardware and not even realize it is a router. What defines a router is not its shape, color, size or manufacturer, but its job function of routing data packets between computers. A cable modem which routes data between your PC and your ISP can be considered a router. In its most basic form, a router could simply be one of two computers running the Windows 98 (or higher) operating system connected together using ICS (Internet Connection Sharing). In this scenario, the computer that is connected to the Internet is acting as the router for the second computer to obtain its Internet connection.

Going a step up from ICS, we have a category of hardware routers that are used to perform the same basic task as ICS, albeit with more features and functions. Often called broadband or Internet connection sharing routers, these routers allow you to share one Internet connection between multiple computers.

Broadband or ICS routers will look a bit different depending on the manufacturer or brand, but wired routers are generally a small box-shaped hardware device with ports on the front or back into which you plug each computer, along with a port to plug in your broadband modem. These connection ports allow the router to do its job of routing the data packets between each of the the computers and the data going to and from the Internet.

Depending on the type of modem and Internet connection you have, you could also choose a router with phone or fax machine ports. A wired Ethernet broadband router will typically have a built-in Ethernet switch to allow for expansion. These routers also support NAT (network address translation), which allows all of your computers to share a single IP address on the Internet. Internet connection sharing routers will also provide users with much needed features such as an SPI firewall or serve as a a DHCP Server.

Wireless broadband routers look much the same as a wired router, with the obvious exception of the antenna on top, and the lack of cable running from the PCs to the router when it is all set up. Creating a wireless network adds a bit more security concerns as opposed to wired networks, but wireless broadband routers do have extra levels of embedded security. Along with the features found in wired routers, wireless routers also provide features relevant to wireless security such as Wi-Fi Protected Access (WPA) and wireless MAC address filtering. Additionally, most wireless routers can be configured for "invisible mode" so that your wireless network cannot be scanned by outside wireless clients. Wireless routers will often include ports for Ethernet connections as well. For those unfamiliar with WiFi and how it works, it is important to note that choosing a wireless router may mean you need to beef up your Wi-Fi knowledge-base. After a wireless network is established, you may possibly need to spend more time on monitoring and security than one would with a wired LAN.

Wired and wireless routers and the resulting network can claim pros and cons over each other, but they are somewhat equal overall in terms of function and performance. Both wired and wireless routers have high reliability and reasonably good security (without adding additional products). However —and this bears repeating — as we mentioned you may need to invest time in learning more about wireless security. Generally, going wired will be cheaper overall, but setting up the router and cabling in the computers is a bit more difficult than setting up the wireless network. Of course, mobility on a wired system is very limited while wireless offers outstanding mobility features.

MAC Addresses
The chances are very good that you'll never see the MAC address for any of your equipment because the software that helps your computer communicate with a network takes care of matching the MAC address to a logical address. The logical address is what the network uses to pass information along to your computer.
If you'd like to see the MAC address and logical address used by the Internet Protocol (IP) for your Windows computer, you can run a small program that Microsoft provides. Go to the "Start" menu, click on "Run," and in the window that appears, type WINIPCFG (IPCONFIG/ALL for Windows 2000/XP). When the gray window appears, click on "More Info" and you'll get this sort of information:

There's a lot of information here that will vary depending on exactly how your connection to the Internet is established, but the physical address is the MAC address of the adapter queried by the program. The IP address is the logical address assigned to your connection by your ISP or network administrator. You'll see the addresses of other servers, including the DNS servers that keep track of all the names of Internet sites (so you can type "www.howstuffworks.com" rather than "216.27.61.189") and the gateway server that you connect to in order to reach the Internet. When you've finished looking at the information, click OK. (Note: For security reasons, some of the information about this connection to the Internet has been changed. You should be very careful about giving your computer's information to other people -- with your address and the right tools, an unscrupulous person could, in some circumstances, gain access to your personal information and control your system through a "Trojan Horse" program.

Understanding the Protocols
The first and most basic job of the router is to know where to send information addressed to your computer. Just as the mail handler on the other side of the country knows enough to keep a birthday card coming toward you without knowing where your house is, most of the routers that forward an e-mail message to you don't know your computer's MAC address, but they know enough to keep the message flowing.
Routers are programmed to understand the most common network protocols. That means they know the format of the addresses, how many bytes are in the basic package of data sent out over the network, and how to make sure all the packages reach their destination and get reassembled. For the routers that are part of the Internet's main "backbone," this means looking at, and moving on, millions of information packages every second. And simply moving the package along to its destination isn't all that a router will do. It's just as important, in today's computerized world, that they keep the message flowing by the best possible route.

In a modern network, every e-mail message is broken up into small pieces. The pieces are sent individually and reassembled when they're received at their final destination. Because the individual pieces of information are called packets and each packet can be sent along a different path, like a train going through a set of switches, this kind of network is called a packet-switched network. It means that you don't have to build a dedicated network between you and your friend on the other side of the country. Your e-mail flows over any one of thousands of different routes to get from one computer to the other.

Depending on the time of day and day of the week, some parts of the huge public packet-switched network may be busier than others. When this happens, the routers that make up this system will communicate with one another so that traffic not bound for the crowded area can be sent by less congested network routes. This lets the network function at full capacity without excessively burdening already-busy areas. You can see, though, how Denial of Service attacks (described in the next section), in which people send millions and millions of messages to a particular server, will affect that server and the routers forwarding message to it. As the messages pile up and pieces of the network become congested, more and more routers send out the message that they're busy, and the entire network with all its users can be affected.

Backbone of the Internet
In order to handle all the users of even a large private network, millions and millions of traffic packets must be sent at the same time. Some of the largest routers are made by Cisco Systems, Inc., a company that specializes in networking hardware. Cisco's Gigabit Switch Router 12000 series of routers is the sort of equipment that is used on the backbone of the Internet. These routers use the same sort of design as some of the most powerful supercomputers in the world, a design that ties many different processors together with a series of extremely fast switches. The 12000 series uses 200-MHz MIPS R5000 processors, the same type of processor used in the workstations that generate much of the computer animation and special effects used in movies. The largest model in the 12000 series, the 12016, uses a series of switches that can handle up to 320 billion bits of information per second and, when fully loaded with boards, move as many as 60 million packets of data every second. Beyond the computing power of the processors, these routers can handle so much information because they are very highly specialized. Relieved of the burden of displaying 3-D graphics and waiting for mouse input, modern processors and software can cope with amazing amounts of information.
Even with the computing power available in a very large router, how does it know which of the many possibilities for outbound connection a particular packet should take? The answer lies back in the configuration table. The router will scan the destination address and match that IP address against rules in the configuration table. The rules will say that packets in a particular group of addresses (a group that may be large or small, depending on precisely where the router is) should go in a specific direction. Next the router will check the performance of the primary connection in that direction against another set of rules. If the performance of the connection is good enough, the packet is sent, and the next packet handled. If the connection is not performing up to expected parameters, then an alternate is chosen and checked. Finally, a connection will be found with the best performance at a given moment, and the packet will be sent on its way. All of this happens in a tiny fraction of a second, and this activity goes on millions of times a second, around the world, 24 hours every day.

Knowing where and how to send a message is the most important job of a router. Some simple routers do this and nothing more. Other routers add additional functions to the jobs they perform. Rules about where messages from inside a company may be sent and from which companies messages are accepted can be applied to some routers. Others may have rules that help minimize the damage from "denial of service" attacks. The one constant is that modern networks, including the Internet, could not exist without the router.